1. Field of the Invention
The present invention relates to a printing method, which creates at least three different types of dots and thereby enables printing of a multi-tone image, as well as a printing apparatus to actualize the printing method. The invention also pertains to a recording medium on which a program for actualizing the printing method is recorded.
2. Description of the Related Art
An ink jet printer proposed as an output apparatus of the computer creates dots with inks of different colors spouted from a plurality of nozzles mounted on a head and thereby records a multi-color image. The ink jet printer has widely been used to print an image processed by the computer in a multi-color, multi-tone manner. In this printer, each pixel typically has only two tones, that is, the dot-on state and the dot-off state. An image is accordingly printed after the halftone processing, which is the image processing performed to enable the tone of the original image data to be expressed by the dispersibility of dots.
In order to enrich the tone expression, the recently proposed ink jet printer is a multi-valuing printer that enables the tone expression of each pixel in three or a greater number of values. One example of the multi-valuing printer varies the dot diameter and the ink density and thus enables three or a greater number of different densities for each dot. Another example of the multi-valuing printer creates a plurality of dots in each pixel in an overlapping manner to enable the multi-tone expression. In these printers, the halftone processing is required since the tone of the original image data can not be expressed sufficiently in each pixel unit.
Typical methods applicable for the halftone processing include the dither method and the error diffusion method. The dither method determines the on-off state of the respective dots based on the results of the comparison between a threshold value corresponding to each pixel read from a dither matrix and the tone value of the image data. The error diffusion method diffuses a density error arising due to the determination for the on-off state of the dot with respect to a certain pixel into peripheral pixels and corrects the tone data of these peripheral pixels. The error diffusion method then determines the on-off state of the respective dots based on the results of the comparison between the corrected tone data and a predetermined threshold value. The advantage of the dither method is the high-speed processing, whereas the advantage of the error diffusion method is the excellent picture quality.
In the multi-valuing printer that enables tone expression in three or a greater number of values, a plurality of different types of dots can be created for an identical hue. The halftone processing generally determines the on-off state of the respective dots individually. In order to enable the smooth expression of multiple tones and realize the printing of high picture quality, it is desirable to prevent the dots of the identical hue from being created in the same pixel in an overlapping manner. The technique of halftone processing that takes into account this point is, for example, described in JAPANESE PATENT LAID-OPEN GAZETTE No. 10-157167.
The proposed technique of halftone processing adopts the dither method in the following manner to determine the on-off state of the deep dot having a higher density and the light dot having a lower density. A recording ratio of the deep dot and a recording ratio of the light dot corresponding to the tone data of each pixel are read from a table provided in advance. The determination for the on-off state is carried out in the sequence of the deep dot and the light dot, based on the result of the comparison between a threshold value read from an identical dither matrix and the recording ratio. The result of the comparison between the threshold value and the sum of the recording ratio of the deep dot and the recording ratio of the light dot is used in the determination for the on-off state of the light dot. The light dot is set in the off state in the pixel where the deep dot has been set in the on state. This technique enables the deep dot and the light dot to be recorded in different pixels while maintaining the required recording ratios of the deep dot and the light dot.
There has been, however, little consideration on the sequence of making the dots subjected to the determination for the on-off state and the selection of the adequate method for the halftone processing in the multi-valuing printer that is provided with at least two inks having an identical hue but different densities and enables at least two different types of dots having different ink weights to be created. One example of the multi-valuing printer is provided with two inks of different densities, that is, the deep ink and the light ink, and enables at least two different types of dots having different ink weights to be created by each ink.
In order to realize the printing of high picture quality, it is desirable to create the dots of the identical hue in a sufficiently dispersing manner. The dots of the identical hue, however, often require different dispersibilities. The dots created by the inks of different densities are generally formed by separate heads. Creation of these dots in an identical pixel thus does not significantly affect the printing speed. Creation of the dots having different ink weights by an identical ink in an identical pixel, on the other hand, requires the head to be driven twice or more times for the same pixel, thereby significantly lowering the printing speed. The requirement for the sufficient dispersibility of the dots is thus especially high for the dots created by the identical ink.
An increase in number of different dots subjected to the determination for the on-off state in each pixel results in the longer time required for the halftone processing. Application of the dither method for the halftone processing does not ensure the high picture quality, while enabling the high-speed processing. The primary object of the multivaluing printer enriches the tone expression and realizes the printing of high picture quality. The deterioration of the picture quality is thus not a negligible problem in the multi-valuing printer. Application of the error diffusion process for the halftone processing ensures the sufficient picture quality, but undesirably lengthens the time required for the processing.
There has been no proposed technique that selectively applies the adequate method for the halftone processing of the respective dots created by the multi-valuing printer and specifies the sequence of making the respective dots subjected to the halftone processing, by taking into account these advantages and disadvantages. The problems discussed above arise not only in the printers but in a variety of printing apparatuses that can express the density of each pixel in three or a greater number of values by the inks of different densities and the dots having different ink weights.
The object of the present invention is thus to provide a technique that determines the on-off state of the respective dots in an adequate sequence and enables the printing of high picture quality in the multi-valuing printer that is provided with at least two inks having an identical hue but different densities and enables at least two different types of dots having different ink weights to be created by at least one ink among the at least two inks.
At least part of the above and the other related objects is realized by a first method of printing a multi-tone image as a distribution of dots created with a head wherein the head is provided with at least two different inks that have an identical hue but different densities and enables at least three different types of dots, which include at least two different types of dots that have different ink weights and are created by at least one ink among the at least two different inks, to be created on a printing medium. The first method includes the steps of: (a) inputting tone data of an image to be printed; (b) successively determining on-off state of the at least three different types of dots with respect to each pixel in a preset sequence, the preset sequence including a specific order of ink weight in which the at least two different types of dots created by the identical ink are consecutively subjected to the determination for the on-off state; and (c) driving the head based on results of the determination carried out in the step (b), so as to create the respective types of dots.
The step (b) includes the steps of: (b1) setting a recording ratio regarding each of the at least three different types of dots, based on the tone data; (b2) comparing the recording ratio set in the step (b1) for a specific dot, which is a first object of the determination, with a threshold value read from a dither matrix that has been provided in advance and determining on-off state of the specific dot based on a result of the comparison; and (b3) comparing a corrected recording ratio with the threshold value and determining on-off state of another dot, which is a second or subsequent object of the determination, based on a result of the comparison with respect to pixels in which all the dots previously subjected to the determination for the on-off state are set in the off state, the corrected recording ratio being obtained by correcting the recording ratio set in the step (b1) for the another dot with the recording ratios of all the dots previously subjected to the determination for the on-off state.
In the first method of the present invention, the step (b), which includes the steps (b1) through (b3), determines the on-off state of the respective dots according to the input tone data, while effectively preventing different types of dots from being created in an identical pixel in an overlapping manner. In one example, a dot is set in the on state when the recording ratio is greater than the threshold value stored in the dither matrix. With respect to the specific dot, which is a first target subjected to the determination for the on-off state, the dot is set in the on state in the pixels having relatively small threshold values read from the dither matrix corresponding to the recording ratio. The method determines the on-off state of another dot, which is a second target of the determination, in the pixels where the specific dot is in the off state. The pixels where the specific dot or the first target of the determination are in the off state have relatively large threshold values read from the dither matrix. If the on-off state of the second target dot is determined only according to the recording ratio of the second target dot, the probability of the on state of the second target dot is unjustifiably lowered. The step (b3) accordingly compares the sum of the recording ratio of the second target dot and the recording ratio of the first target dot with the threshold value. This prevents the unjustifiably high probability of non-creation of the second target dot and ensures a desired recording ratio. This arrangement determines the on-off state of the second target dot, while effectively preventing the second target dot from being created in an overlapping manner in the pixel where the first target dot already exists. This processing is executed for third and subsequent target dots.
In the step (b3), the method applied to make the recording ratios of all the dots previously subjected to the determination for the on-off state reflect on the recording ratio of a target dot currently processed depends upon when the dot is set in the on state, that is, when the recording ratio is greater than the threshold value or when the recording ratio is smaller than the threshold value. In the case where the dot is set in the on state when the recording ratio is greater than the threshold value, the recording ratios of all the dots previously subjected to the determination for the on-off state are added to the recording ratio of the target dot currently processed. In the case where the dot is set in the on state when the recording ratio is smaller than the threshold value, on the other hand, the recording ratios of all the dots previously subjected to the determination for the on-off state are subtracted from the recording ratio of the target dot currently processed.
The steps (b1) through (b3) are carried out independently of the type of ink. By way of example, it is assumed that four different types of dots are created by using two inks of different weights, that is, the deep ink and the light ink, in two different ink weights, that is, the large dot and the small dot. The determination for the on-off state is carried out in the sequence of xe2x80x98the large dot by the deep inkxe2x80x99, xe2x80x98the small dot by the deep inkxe2x80x99, xe2x80x98the large dot by the light inkxe2x80x99, and xe2x80x98the small dot by the light inkxe2x80x99. According to the first method of the present invention, for example, the on-off state of the large dot by the light ink is determined by taking into account the recording ratios of the previously processed dots, that is, the large dot by the deep ink and the small dot by the deep ink. In this manner, the on-off state of the target dot is determined by taking into account the recording ratios of the previously processed dots, whether or not the type of the ink is identical. This arrangement effectively prevents the dots created by the inks of different densities from being created in an identical pixel in an overlapping manner.
In the first method of the present invention, the step (b) successively determines the on-off state of the respective dots in a preset sequence. In this case, the dots created by the identical ink are consecutively subjected to the determination for the on-off state in the order of ink weight. The four different types of dots are, for example, subjected to the determination for the on-off state in the sequence specified above. The determination may, however, be carried out in a different sequence, that is, in the sequence of xe2x80x98the large dot by the light inkxe2x80x99, xe2x80x98the small dot by the light inkxe2x80x99, xe2x80x98the large dot by the deep inkxe2x80x99, and xe2x80x98the small dot by the deep inkxe2x80x99. The sequence is independent of the density expressed per unit area by each dot. The process of consecutively determining the on-off state of the dots created by the identical ink has the significant advantage, that is, the easy regulation of the dot creation even in the case where the dots should be created in an overlapping manner.
A variety of settings are possible for the recording ratios of the dots. The total of the recording ratios of the dots may exceed 100%. When the total recording ratio exceeds 100%, the dots of an identical hue are created in the same pixel in an overlapping manner. The dot-forming elements are provided separately for the dots that have the identical hue but are created by the inks of different densities. Creating these dots in an overlapping manner thus does not significantly affect the printing speed. When the dots by the identical ink are created in the same pixel in an overlapping manner, on the other hand, the same dot-forming element is driven twice or more for one identical pixel. This undesirably lowers the printing speed. The first method of the present invention consecutively determines the on-off state of the dots created by the identical ink. This ensures the easy regulation of these dots. Even when dots should be created in some pixels in an overlapping manner, the arrangement of the first method reduces the possibility of creating the dots by the identical ink in an overlapping manner and thereby prevents the printing speed from being significantly lowered.
In the step (b), the on-off state of the dots may be determined in an arbitrary sequence, as long as the dots created by the identical ink are consecutively subjected to the determination for the on-off state. One possible modification carries out the determination for the on-off state of the dots created by the lower-density ink before the determination for the on-off state of the dots created by the higher-density ink. The dots created by the identical ink may be subjected to the determination for the on-off state in a descending order of the ink weight or in an ascending order of the ink weight, or irrespective of the ink weight.
The sequence of the determination for the on-off state of the dot is not fixed but may be set adequately as discussed above. In the first method of the present invention, for example, it is preferable that the step (b) successively determines the on-off state of the at least three different types of dots in a descending order of ink density.
In general, the dots created by the high-density ink are readily recognized with eyes. The sufficient dispersibility of these dots is thus desired to ensure the high picture quality. The determination for the on-off state in the above sequence enables the dots created by the higher-density ink to have the higher degree of freedom in the determination for the on-off state. This arrangement ensures the sufficient dispersibility of the dots and thus improves the picture quality.
In accordance with one preferable application of the first method, the head enables creation of at least four different types of dots, which include dots formed in at least two different ink weights respectively by a deep ink having a higher ink density and a light ink having a lower ink density. In this structure, the step (b) first determines the on-off state of the dots created by the deep ink in a descending order of ink weight and subsequently determines the on-off state of the dots created by the light ink in a descending order of ink weight. The first method of the present invention is, however, naturally not restricted to this application.
The present invention is also directed to a second method of printing a multi-tone image as a distribution of dots created with a head, wherein the head is provided with at least two different inks that have an identical hue but different densities and enables at least three different types of dots, which include at least two different types of dots that have different ink weights and are created by at least one identical ink among the at least two different inks, to be created on a printing medium. The second method includes the steps of: (a) inputting tone data of an image to be printed; (b) dividing the at least three different types of dots according to a preset classification into a plurality of different types of dots having higher density evaluation values and at least one type of dot having a lower density evaluation value; (c) successively determining on-off state of the plurality of different types of dots having the higher density evaluation values with respect to each pixel in a preset sequence, the preset sequence including a specific order of ink weight in which the at least two different types of dots created by the identical ink are consecutively subjected to the determination for the on-off state; (d) determining on-off state of the at least one type of dot having the lower density evaluation value by an error diffusion method based on correction data, the correction data being obtained by correcting the tone data with density errors occurring due to the on-off state of the plurality of dots having the higher density evaluation values; and (e) driving the head based on results of the determinations carried out in the step (c) and the step (d), so as to create the respective types of dots.
The step (c) includes the steps of: (c1) setting a recording ratio regarding each of the plurality of different types of dots having the higher density evaluation values, based on the tone data; (c2) comparing the recording ratio set in the step (c1) for a specific dot, which is a first object of the determination, with a threshold value read from a dither matrix that has been provided in advance and determining on-off state of the specific dot based on a result of the comparison; and (c3) comparing a corrected recording ratio with the threshold value and determining on-off state of another dot, which is a second or subsequent object of the determination, based on a result of the comparison with respect to pixels in which all the dots previously subjected to the determination for the on-off state are set in the off state, the corrected recording ratio being obtained by correcting the recording ratio set in the step (c1) for the another dot with the recording ratios of all the dots previously subjected to the determination for the on-off state.
The second method of the present invention divides the dots into two groups according to the density evaluation value. The density evaluation value represents the density expressible of the dot per unit area. The second method applies the different methods of determination to the dots having the lower density evaluation values and the dots having the higher density evaluation values. The error diffusion method is adopted in the determination for the on-off state of the dots having the lower density evaluation values, whereas the dither method is adopted in the determination for the on-off state of the dots having the higher density evaluation values. As discussed previously in the first method of the present invention, the determination for the on-off state by the dither method in the step (b) ensures the dispersibility of the dots having the identical hue.
When the dither method is adopted in the determination for the on-off state of the dot, there may be a relatively large local error occurring as the difference between the density expressed according to the result of the determination and the density to be expressed according to the tone data. Application of the error diffusion method to the determination for the on-off state of the dot effectively reduces the local error. The second method of the present invention, the error diffusion method that takes advantage of the density errors arising due to the on-off state of the other dots is adopted in the determination for the on-off state of the dot having the lower density evaluation value. This arrangement effectively reduces the local density error and thus enables the printing of high picture quality.
An arbitrary number of dots having the lower density evaluation values may be selected as the dot having the lower density evaluation value in the second method among the at least three different types of dots created by the head. It is, however, necessary to cause a plurality of dots having the higher density evaluation values to be processed by the dither method. The dots having the lower density evaluation values are not restricted to the dots created by the identical ink.
For example, the at least one type of dot having the lower density evaluation value may be a dot having a lowest density evaluation value among the at least three different types of dots created with the head.
The disadvantage of the error diffusion method is a long processing time. The second method of the present invention adopts the error diffusion method only to the dot having the lowest density evaluation value. This arrangement thus does not significantly increase the processing time while realizing the printing of high picture quality. The dot having the lowest density evaluation value is generally conspicuous with eyes. Application of the error diffusion method to this dot advantageously cancels the local density error without making the dot conspicuous.
Like the first method discussed above, the second method of the present invention carries out the determination for the on-off state of a target dot according to the dither method, by taking into account the recording ratios of all the dots previously subjected to the determination for the on-off state, whether or not the respective dots are created by the identical ink. In accordance with one preferable application of the second method, the step (c) accordingly carries out the determination for the on-off state of the respective dots with a single dither matrix, whether or not the respective dots are created by the identical ink. In another example, the recording ratios of the dots created by the identical ink and previously subjected to the determination for the on-off state may be made to reflect on the recording ratio of the currently processed target pixel. It is here desirable that different dither matrixes are used for different inks, in order to ensure the sufficient dispersibility of the dots created by the different inks. It is possible to provide completely different dither matrixes for the respective inks, but the following application is more practical and preferable.
In accordance with one preferable application of the second method, the step (c) further includes the step of: (c0) providing a new dither matrix, which includes threshold values in a different arrangement specified for each of the at least two different inks, from a dither matrix stored in advance. The step (c) accordingly performs the step (c2) and the step (c3) to carry out the determination for the on-off state with the dither matrix corresponding to each of the at least two different inks, with respect to each dot created by the ink.
The method of this structure enables a dither matrix corresponding to each ink to be newly prepared from the dither matrix that has been stored in advance. This arrangement desirably saves the required capacity of the memory for storing the dither matrixes. The preparation of the dither matrix corresponding to each ink may be implemented by changing the mapping of the dither matrix to the pixels for each ink. It is not necessary to store the newly prepared dither matrix in the memory, before the dither matrix is used in the determination for the on-off state of the dot.
The above application causes the recording ratios of the dots created by the identical ink and previously subjected to the determination for the on-off state to be reflected separately on the recording ratio of the currently processed target pixel and exerts the following effects. In the structure that makes the recording ratios of all the dots previously processed reflect on the recording ratio of the target pixel, when the total of the recording ratios of the dots exceeds 100%, the process of determining the on-off state of the dot may become undesirably complicated. The structure that makes the recording ratios of the dots created by the identical ink and processed previously reflect on the recording ratio of the target pixel, on the other hand, effectively prevents the dots by the identical ink from being created in an identical pixel in an overlapping manner, whereas allowing the dots by the different inks to be created in an identical pixel in an overlapping manner. The latter structure enables the parallel determination for the on-off state of the dots for the respective inks, thereby ensuring the higher-speed processing. This arrangement also ensures the easy regulation of the recording ratios of the dots created by the identical ink, thereby enabling the amount of ink spout to be controlled by the simple procedure.
The second method of the present invention is applicable to the printing apparatus that can create dots in a variety of ink weights with inks of varies densities. In accordance with one concrete application of the second method, for example, the head enables creation of at least four different types of dots, which include dots formed in at least two different ink weights respectively by a deep ink having a higher ink density and a light ink having a lower ink density. The at least one type of dot having the lower density evaluation value is a dot that has a lowest density evaluation value and is created by the light ink. In this structure, the step (c) excludes the dot that has the lowest density evaluation value and is created by the light ink, first determines the on-off state of the dots created by the deep ink in a descending order of ink weight, and subsequently determines the on-off state of the dots created by the light ink in a descending order of ink weight. The second method is, however, naturally not restricted to this concrete application.
The present invention is further directed to a first printing apparatus that enables a multi-tone image to be printed as a distribution of dots created with a head, wherein the head is provided with at least two different inks that have an identical hue but different densities and enables at least three different types of dots, which include at least two different types of dots that have different ink weights and are created by at least one ink among the at least two different inks, to be created on a printing medium. The first printing apparatus includes: an input unit that inputs tone data of an image to be printed; a dither determination unit that successively determines on-off state of the at least three different types of dots with respect to each pixel in a preset sequence, the preset sequence including a specific order of ink weight in which the at least two different types of dots created by the identical ink are consecutively subjected to the determination for the on-off state; and a dot creation unit that drives the head based on results of the determination carried out by the dither determination unit, so as to create the respective types of dots.
The dither determination unit includes: a memory unit in which a relationship between tone data and a recording ratio regarding each of the at least three different types of dots is stored; a setting unit that refers to the memory unit and sets the recording ratio of each of the at least three different types of dots corresponding to the input tone data; and a dot creation determination unit that compares a corrected recording ratio of a target dot with a threshold value stored in a dither matrix that has been provided in advance, the corrected recording ratio being obtained by correcting the recording ratio of the target dot set by the setting unit with the recording ratios of all the dots previously subjected to the determination for the on-off state, the dot creation determination unit then determining on-off state of the target dot with respect to pixels in which all the dots previously subjected to the determination for the on-off state are set in the off state, based on a result of the comparison.
In accordance with one preferable application of the first printing apparatus, the dither determination unit successively determines the on-off state of the at least three different types of dots in a descending order of ink density.
In accordance with another preferable application of the first printing apparatus, the head enables creation of at least four different types of dots, which include dots formed in at least two different ink weights respectively by a deep ink having a higher ink density and a light ink having a lower ink density. In this structure, the dither determination unit first determines the on-off state of the dots created by the deep ink in a descending order of ink weight and subsequently determines the on-off state of the dots created by the light ink in a descending order of ink weight.
The present invention is also directed to a second printing apparatus that enables a multi-tone image to be printed as a distribution of dots created with a head, wherein the head is provided with at least two different inks that have an identical hue but different densities and enables at least three different types of dots, which include at least two different types of dots that have different ink weights and are created by at least one ink among the at least two different inks, to be created on a printing medium. The second printing apparatus includes: a storage unit that stores divisions, which are obtained by dividing the at least three different types of dots according to a preset classification into a plurality of different types of dots having higher density evaluation values and at least one type of dot having a lower density evaluation value; an input unit that inputs tone data of an image to be printed; a dither determination unit that successively determines on-off state of the plurality of different types of dots having the higher density evaluation values with respect to each pixel in a preset sequence, the preset sequence including a specific order of ink weight in which the at least two different types of dots created by the identical ink are consecutively subjected to the determination for the on-off state; an error diffusion determination unit that determines on-off state of the at least one type of dot having the lower density evaluation value by an error diffusion method based on correction data, the correction data being obtained by correcting the tone data with density errors occurring due to the on-off state of the plurality of dots having the higher density evaluation values; and a dot creation unit that drives the head based on results of the determinations carried out by the dither determination unit and the error diffusion determination unit, so as to create the respective types of dots.
The dither determination unit includes: a memory unit in which a relationship between tone data and a recording ratio regarding each of the plurality of different types of dots having the higher density evaluation values is stored; a setting unit that refers to the memory unit and sets the recording ratio of each of the plurality of different types of dots having the higher density evaluation values corresponding to the input tone data; and a dot creation determination unit that compares a corrected recording ratio of a target dot with a threshold value stored in a dither matrix that has been provided in advance, the corrected recording ratio being obtained by correcting the recording ratio of the target dot set by the setting unit with the recording ratios of all the dots previously subjected to the determination for the on-off state, the dot creation determination unit then determining on-off state of the target dot with respect to pixels in which all the dots previously subjected to the determination for the on-off state are set in the off state, based on a result of the comparison.
In the second printing apparatus, it is preferable that the at least one type of dot having the lower density evaluation value is a dot having a lowest density evaluation value among the at least three different types of dots created with the head.
In accordance with one preferable application of the second printing apparatus, the dither determination unit carries out the determination for the on-off state of the respective dots with a single dither matrix, whether or not the respective dots are created by the identical ink.
In accordance with another preferable application of the second printing apparatus, the dither determination unit further includes: a dither matrix generation unit that provides a new dither matrix, which includes threshold values in a different arrangement specified for each of the at least two different inks, from a dither matrix stored in advance; and a decision unit that carries out the determination for the on-off state with the dither matrix corresponding to each of the at least two different inks, with respect to each dot created by the ink.
In accordance with still another preferable application of the second printing apparatus, the head enables creation of at least four different types of dots, which include dots formed in at least two different ink weights respectively by a deep ink having a higher ink density and a light ink having a lower ink density. The at least one type of dot having the lower density evaluation value is a dot that has a lowest density evaluation value and is created by the light ink. In this structure, the dither determination unit excludes the dot that has the lowest density evaluation value and is created by the light ink, first determines the on-off state of the dots created by the deep ink in a descending order of ink weight, and subsequently determines the on-off state of the dots created by the light ink in a descending order of ink weight.
These printing apparatuses can actualize one of the methods of the present invention discussed above. In the first printing apparatus, the input unit, the dither determination unit, and the dot creation unit may be incorporated in the same casing or in different casings. Similarly, in the second printing apparatus, the input unit, the dither determination unit, the error diffusion determination unit, and the dot creation unit may be incorporated in the same casing or in different casings. The printing apparatus may have a plurality of casings, which respectively include some of the required elements in an arbitrary combination.
The method of the present invention is actualized by the printing apparatus that drives with the data processed by the computer according to a preset program. The method of the present invention is accordingly realized by a recording medium in which such a program is recorded.
The present invention is thus directed to a first recording medium in which a program for enabling a multi-tone image to be printed by a printing apparatus is recorded in a computer readable manner, wherein the printing apparatus is provided with at least two different inks that have an identical hue but different densities and enables at least three different types of dots, which include at least two different types of dots that have different ink weights and are created by at least one ink among the at least two different inks, to be created on a printing medium. The program includes: a program code that causes a computer to successively determine on-off state of the at least three different types of dots with respect to each pixel in a preset sequence, the preset sequence including a specific order of ink weight in which the at least two different types of dots created by the identical ink are consecutively subjected to the determination for the on-off state.
The program code causes the computer to actualize the functions of: setting a recording ratio regarding each of the at least three different types of dots, based on tone data of an original image; comparing the recording ratio of a specific dot, which is a first object of the determination, with a threshold value read from a dither matrix that has been provided in advance and determining on-off state of the specific dot based on a result of the comparison; and comparing a corrected recording ratio with the threshold value and determining on-off state of another dot, which is a second or subsequent object of the determination, based on a result of the comparison with respect to pixels in which all the dots previously subjected to the determination for the on-off state are set in the off state, the corrected recording ratio being obtained by correcting the recording ratio of the another dot with the recording ratios of all the dots previously subjected to the determination for the on-off state.
In accordance with one preferable application of the first recording medium, the program code causes the computer to successively determine the on-off state of the at least three different types of dots in a descending order of ink density.
In accordance with another preferable application of the first recording medium, the printing apparatus enables creation of at least four different types of dots, which include dots formed in at least two different ink weights respectively by a deep ink having a higher ink density and a light ink having a lower ink density. In this structure, the program code causes the computer to first determine the on-off state of the dots created by the deep ink in a descending order of ink weight and subsequently determine the on-off state of the dots created by the light ink in a descending order of ink weight.
The present invention is further directed to a second recording medium in which a program for enabling a multi-tone image to be printed by a printing apparatus is recorded in a computer readable manner, wherein the printing apparatus is provided with at least two different inks that have an identical hue but different densities and enables at least three different types of dots, which include at least two different types of dots that have different ink weights and are created by at least one ink among the at least two different inks, to be created on a printing medium. The program includes: a first program code that causes a computer to divide the at least three different types of dots according to a preset classification into a plurality of different types of dots having higher density evaluation values and at least one type of dot having a lower density evaluation value; a second program code that causes the computer to successively determine on-off state of the plurality of different types of dots having the higher density evaluation values with respect to each pixel in a preset sequence, the preset sequence including a specific order of ink weight in which the at least two different types of dots created by the identical ink are consecutively subjected to the determination for the on-off state; and a third program code that causes the computer to determine on-off state of the at least one type of dot having the lower density evaluation value by an error diffusion method based on correction data, the correction data being obtained by correcting the tone data with density errors occurring due to the on-off state of the plurality of dots having the higher density evaluation values.
The second program code causes the computer to actualize the functions of: setting a recording ratio regarding each of the plurality of different types of dots having the higher density evaluation values, based on tone data of an original image; comparing the recording ratio of a specific dot, which is a first object of the determination, with a threshold value read from a dither matrix that has been provided in advance and determining on-off state of the specific dot based on a result of the comparison; and comparing a corrected recording ratio with the threshold value and determining on-off state of another dot, which is a second or subsequent object of the determination, based on a result of the comparison with respect to pixels in which all the dots previously subjected to the determination for the on-off state are set in the off state, the corrected recording ratio being obtained by correcting the recording ratio of the another dot with the recording ratios of all the dots previously subjected to the determination for the on-off state.
In the second recording medium of the present invention, it is preferable that the at least one type of dot having the lower density evaluation value is a dot having a lowest density evaluation value among the at least three different types of dots created by the printing apparatus.
In accordance with one preferable application of the second recording medium, the second program code causes the computer to carry out the determination for the on-off state of the respective dots with a single dither matrix, whether or not the respective dots are created by the identical ink.
In accordance with another preferable application of the second recording medium, the second program code further causes the computer to actualize the functions of: providing a new dither matrix, which includes threshold values in a different arrangement specified for each of the at least two different inks, from a dither matrix stored in advance; and carrying out the determination for the on-off state with the dither matrix corresponding to each of the at least two different inks, with respect to each dot created by the ink.
In accordance with still another preferable application of the second recording medium, the printing apparatus enables creation of at least four different types of dots, which include dots formed in at least two different ink weights respectively by a deep ink having a higher ink density and a light ink having a lower ink density. The at least one type of dot having the lower density evaluation value is a dot that has a lowest density evaluation value and is created by the light ink. In this structure, the second program code causes the computer to exclude the dot that has the lowest density evaluation value and is created by the light ink, first determine the on-off state of the dots created by the deep ink in a descending order of ink weight, and subsequently determine the on-off state of the dots created by the light ink in a descending order of ink weight.
The computer executes the program recorded in any one of the above recording media, so as to actualize the printing method of the present invention discussed above. Available examples of the recording media include flexible disks, CD-ROMs, magneto-optic discs, IC cards, ROM cartridges, punched cards, prints with barcodes or other codes printed thereon, internal storage devices (memories like a RAM and a ROM) and external storage devices of the computer, and a variety of other computer readable media. Still another application is a program supply apparatus that supplies a computer program, which causes the computer to actualize the multi-valuing functions of the image processing apparatus discussed above, to the computer via a communications path.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.